This invention relates to an improved mounting or back-up die for use in fastening a pierce nut to a metal sheet.
Although the shapes and sizes may vary, a typical conventional pierce nut comprises a flat wafer-like piece of metal having a lower portion formed as a punch, for punching a hole through sheet metal, and an integral upper, head portion formed to overlay the punched-hole in the sheet metal. At least a portion of the sheet metal defining the edge of the punched out hole, is deformed into a channel or area provided in the pierce nut for clinching the nut and the sheet together. A threaded opening extends through the pierce nut, for receiving a bolt or screw, after the nut is mounted upon a metal sheet.
Commonly, a series of pierce nuts are arranged edge to edge, in a long chain, with the nuts connected together by a wire or strip of metal which holds the nuts together. The chain of pierce nuts is fed through a machine which includes a pressure applying piston or ram that applies pressure to the head of a pierce nut. The pressure is sufficient to force the punch end of the nut, which is placed upon a metal sheet, to penetrate the sheet and punch out a slug of material. The slug is removed, by gravity, leaving the punch end of the nut in the punched hole. In order to support the sheet metal, as well as to deform portions thereof for interlocking with the pierce nut, a mounting or back-up die is positioned beneath the sheet metal.
One form of conventional mounting or back-up die comprises a cylindrical body having a central opening, through which the punched out slug of sheet material may pass after being punched from the sheet by the pierce nut. The die body has a sheet support end which engages the sheet surface opposite to the surface engaged by the punch portion of the pierce nut. The support end is positioned to engage the sheet around the area of the hole that is made by the pierce nut. The support portion of the die may be in the form of a raised bead which surrounds the central opening formed in the die body through which the slug passes.
Thus, in general, the chain of pierce nuts is fed into the machine whose ram pressure pushes the piece nut through the sheet metal. The sheet metal is supported upon the support end of the die. The punch portion of the pierce nut, punches the hole in the sheet, through which the punch portion extends. Simultaneously to punching, a portion of the sheet material along the edge of the hole is deformed to interlock with the pierce nut for fastening the pierce nut to the sheet.
The mounting die receives considerable pressure and abuse during its use over a substantial period of time. Thus, the bead or support surface of the die frequently wears or breaks to the point where the die must be replaced. Conventionally, when the die bead is no longer suitable for continued use, the entire die must be replaced with a new die. In factory production operations, where numerous pierce nuts are used in manufacturing products, the expenses for replacing dies are relatively high. Therefore, it is desirable to provide a die which lasts longer than conventional dies in production and which resists bead breakage for as long as possible. While harder, sturdier metal materials are available in place of commonly used steel materials in making such dies, typically such materials are too expensive or are too difficult to manufacture to use for such dies. Consequently, it has not been feasible to make a back-up die of more durable or harder materials, such as carbide metals, to reduce wear and breakage.
Therefore, this present invention relates to an improved mounting or back-up die whose support surface lasts longer in use and which can be easily and inexpensively replaced without replacing the entire die or removing the die from the pierce nut mounting equipment.